Digital imaging systems record many images of a scene for various applications. Some applications require accurate knowledge of the positions of points in the scene, in relation to a three-dimensional coordinate system, which, in turn, requires accurate knowledge of the locations of projections of the points in the respective images. One illustrative application is photogrammetry, in which information such as distances between points in a scene, heights of points in the scene above a reference level, and so forth, can be determined from measurements between and among points on the two-dimensional images that were recorded of the scene. Another application is three-dimensional virtual reconstruction of objects in a scene from the two-dimensional images that were recorded of the scene. Such virtual reconstructions can be used, for example, for precise measurement of features of the reconstructed objects.
In some applications, it is desired to measure certain features in a scene with a high degree of accuracy. Such measurements either require that information about the scene be provided to the imaging system, or that the imaging system be able to extract such information from the images in an automated manner. In either case, it is helpful to the imaging system if there are certain features in the scene, as recorded in the images, that can be accurately detected and whose positions and sizes can be accurately measured. These features, which may be referred to as “anchor points” or “targets,” can be planted artificially within the scene to provide reference information for the imaging system. The targets possess predetermined optical characteristics and can readily be automatically differentiated by the imaging system from other objects in the scene. In addition, the imaging system knows the positions in the scene, relative to a three-dimensional coordinate system, of respective ones of the targets. The imaging system, in turn, will need to be able to accurately determine which pixels in the images recorded of the scene relate to respective ones of the targets in the scene. Since the imaging system will need to make a quick and accurate identification of the targets, several problems can arise. In particular, generally special consideration needs to be given to the type of material of which the targets are made, their shapes, and so forth. One technique has been to provide the targets with specific shapes and contours, which can be coded in a predetermined way to ensure that they will stand out and be readily distinguishable from the other objects in the scene. The targets can be made from materials with predetermined surface characteristics, such as diffuse or highly reflective, as long as their shapes and/or reflectance differ considerably from the expected shapes and/or reflectance of the objects in the scene that are to be subject to mensuaration, reconstruction, and so forth.
Another technique involves the use of directional reflecting materials, such as retro-reflective materials for the targets. An object made of a retro-reflective material reflects light that is incident thereon predominately back in the direction of the light source from which the light originates. Types of retro-reflective materials are well known, and are used in, for example, signs, safety reflectors and so forth. The reflection characteristic of the material is generally independent of the angle of incidence of the light on the surface over a relatively wide range of incidence angles. If the objects in the scene that are to be subject to mensuration, reconstruction, and so forth, are not made of retro-reflective materials, the reflective characteristics of their surfaces will differ substantially from the reflective characteristics of the targets, and, if they are properly illuminated, as will be described below, it can be relatively easy for the imaging system to distinguish between the targets and the other objects in the scene.
Yet another technique involves the use of targets that essentially provide holes in the scene by, for example, absorbing light incident thereon or by reflecting the light in such a manner that it will not be directed to the image recording device(s) when images thereof are being recorded.
In order to accurately determine the positions of the targets, they need to be uniformly illuminated so that the appearance of each target will not vary over the field of view or from image to image. However, if the imaging system requires structured illumination, which provides a textured appearance for surfaces that what might otherwise appear relatively featureless, the simultaneous use of structured illumination and uniform illumination will typically reduce-the effect of the structured illumination on the scene, which, in turn, can interfere with the imaging system's ability to perform its mensuration, virtual reconstruction, and so forth, operations. On the other hand, if structured lighting is used alone or predominately to illuminate the scene, including the targets, the appearance of respective ones of the targets can change from image to image, which will make it more difficult for the imaging system to identify the projections of a respective in the various images. In addition, the structured illumination can cause projections of the targets to appear deformed, which can increase the difficulty of accurately determining their locations in the images. Finally, if both structured lighting and uniform illumination are used, but for recording of successive images from what is hoped to be the same direction, problems can arise since one or both of the camera or other device that records the images and the object(s), including the target(s), in the scene can vibrate or otherwise move, which, in turn, can cause inaccuracies in registration between the two images. The time interval between times at which the camera can record successive images can vary based on a number of variables, including, for example, image size and resolution, image buffer download time, and so forth, but often the time interval is long enough for such differences to have adverse effects. This can significantly reduce the accuracy of the mensuration, reconstruction, and/or other operations that the imaging system may be required to perform.